This invention relates to a novel method of manufacturing a synthetic bone-coated material useful for surgical and dental implants.
The mineral fraction of bones and teeth in vertebrates is composed largely of apatites (chemical formula Ca.sub.10 (P0.sub.4).sub.6 0H.sub.2, in addition to carbonate, fluoride, hydroxide, and citrate. Bone crystals belong to the group of hydroxylapatites. These crystals are platelets, or rods, about 8 to 15 angstroms thick, 20-40 angstroms wide, and about 200-400 angstroms long, with a density of about 3.0. This inorganic crystal structure imparts to bone an elastic modulus similar in strength to that of concrete.
Synthetic hydroxylapatites (HA) have been developed and used for a variety of surgical purposes, e.g. to partially fill bone cavities and to promote the growth of new bone about HA fragments. Also, HA coatings have been formed on implant materials to promote the anchoring of the implant in bone.
The use of HA coating for biological implants offers several advantages. Hydroxylapatite (HA) has demonstrated its ability to enhance its integration into bone due to the fact that it biologically binds to natural bone. The deposition of new bone occurs on the HA coating itself leading to a significant increase in the rate at which the surgical site heals.
J. N. Kent, (Abstracts from the 12th Annual Meeting of the Society for Biomaterials, pp. 16, 1986), evaluated the efficacy of HA-coated and non-coated dental implants in dogs. Titanium cylindrical dental implants were coated with a fifty micron thick layer of HA and compared with non-HA coated titanium implants when placed in the anterior mandible and maxilla teeth for 12 weeks. Kent found that none of the non-coated materials adhered to the adjacent bone for any time period, whereas 100% of the HA-coated implants were adherent and could not be removed from the bone. The HA-coated implants demonstrated an intimate bone-implant interface without intervening fibrous tissue. The HA-coated dental implants thus provided an increased stability and retention over polished and grit surfaced cylindrical titanium dental implants.
Various techniques are known for the deposition of HA onto surfaces for use as biological implants. Thomas et al. (12th Annual Meetinq of the Society for Biomaterials. pp. 15, 1986) disclosed that plasma-sprayed HA-coated porous titanium hip implants, that are inserted into adult mongrel dogs, demonstrate increased amounts of bone in growth as compared to non-HA coated implants. The coating was sintered HA about 50 microns thick, which was applied using a plasma spray technique. The bone adjacent to the HA coated implant also appeared to be better organized and had a higher degree of mineralization than the bone adjacent to control implants which lacked the HA coating.
Kay et al. (Abstracts of the 12th Annual Meeting of the Society for Biomaterials, pp. 13, 1986) disclosed the use of HA-coated smooth titanium and cobalt-chrome-molybdenum (Co-Cr-Mo) implants using a modified plasma spray process. Kay et al. report that the coating was of a high density; however, the outermost 15-20% of the coating was less dense due to the nature of the deposition process.
W. R. Lacefield, (Abstracts of the 12th Annual Meeting of the Society for Biomaterials, pp. 12, 1986) compared the coating of sintered alumina, titanium, and the alloys Ti-6Al-4V and Co-Cr-Mo by a dip process and by sputter coating using an Argon beam in a vacuum chamber. The dipping process comprised a repeated dipping of the test specimens in a slurry containing 3-5 mesh HA powder which was then fired at 1100-1200.degree. C. for 1-3 hours. The sputter coating was accomplished by cleaning the materials first by use of an Argon ion beam, followed by sputter coating using a 6 inch diameter target of dense HA placed in the path of Argon ions (typically having energy of 2-3 kev) while the target was arranged on a rotating wheel. The sputter process was continued for 17-20 hours and produced a coating of 0.5-2.2 microns. Lacefield disclosed that dip coating had an adverse effect on the microstructure of the coated materials. This was due to an uncontrolled grain growth on the alumina, titanium and titanium alloy, and a massive carbide precipitation on the Co-Cr-Mo alloy. This led to low bone strengths and fracturing when the implant went from a high temperature (500.degree. C.) to water. The sputter coated material, however, demonstrated a uniform thickness covering all topological features of the substrate and a high integrity of the HA coating. However, X-ray diffraction demonstrated that some sputter-coated implants had coatings which were not crystalline HA, but were primarily an amorphous calcium-phosphate layer.
All of the above prior art techniques suffer from the disadvantage that they form a brittle layer of deposited material which can easily break off. Additionally, the production of a rough and irregular coating by the prior art techniques can lead to irritation of the tissue in the area where the implant is applied, if growth occurs there. Moreover, the prior art techniques cannot be applied to threaded implant configurations such as screws or total hip replacements.
Therefore, what is needed is an improved process for coating materials for use in implants which overcomes the drawbacks and difficulties mentioned above in the prior art processes.